The excitation of valence electrons to the conduction band
is best accomplished when the semiconductor is in the crystalline state, i.e.
when the atoms are arranged in a precise geometrical formation or “lattice.” At
room temperature and low illumination, pure or so-called intrinsic
semiconductors have a high resistively. But the resistively can be greatly
reduced by doping,” i.e. introducing a very small amount of impurity, of the
order of one in a million atoms. There are 2 kinds of doping. Those which have
more valence electrons that the semiconductor itself are called donors and those
which have fewer are termed acceptors [Book 2]. In a silicon crystal, each atom
has 4 valence electrons, which are shared with a neighboring atom to form a
stable tetrahedral structure. Phosphorus, which has 5 valence electrons, is a
donor and causes extra electrons to appear in the conduction band. Silicon so
doped is called n-type [Book 5]. On the other hand, boron, with a valence of 3,
is an acceptor, leaving so-called holes in the lattice, which act like positive
charges and render the silicon p-type[Book 5]. Holes, like electrons, will
remove under the influence of an applied voltage but, as the mechanism of their
movement is valence electron substitution from atom to atom, they are less
mobile than the free conduction electrons [Book 2]. In a n-on-p crystalline
silicon Tran 4 solar cell, a shadow junction is formed by diffusing phosphorus
into a boron-based base. At the junction, conduction electrons from donor atoms
in the n-region diffuse into the p-region and combine with holes in acceptor
atoms, producing a layer of negatively-charged impurity atoms. The opposite
action also takes place, holes from acceptor atoms in the p-region crossing into
the n-region, combining with electrons and producing positively-charged impurity
atoms [Book 4]. The net result of these movements is the disappearance of
conduction electrons and holes from the vicinity of the junction and the
establishment there of a reverse electric field, which is positive on the n-side
and negative on the p-side. This reverse field plays a vital part in the
functioning of the device. The area in which it is set up is called the
depletion area or barrier layer[Book 4]. When light falls on the front surface,
photons with energy in excess of the energy gap interact with valence electrons
and lift them to the conduction band. This movement leaves behind holes, so each
photon is said to generate an electron-hole pair [Book 2].
In the crystalline
silicon, electron-hole generation takes place throughout the thickness of the
cell, in concentrations depending on the irradiance and the spectral composition
of the light. Photon energy is inversely proportional to wavelength. The highly
energetic photons in the ultra-violet and blue part of the spectrum are absorbed
very near the surface, while the less energetic longer wave photons in the red
and infrared are absorbed deeper in the crystal and further from the junction
[Book 4]. Most are absorbed within a thickness of 100 æm. The electrons and
holes diffuse through the crystal in an effort to produce an even distribution.
Some recombine after a lifetime of the order of one millisecond, neutralizing
their charges and giving up energy in the form of heat. Others reach the
junction before their lifetime has expired. There they are separated Tran 5 by
the reverse field, the electrons being accelerated towards the negative contact
and the holes towards the positive [Book 5]. If the cell is connected to a load,
electrons will be pushed from the negative contact through the load to the
positive contact, where they will recombine with holes. This constitutes an
electric current. In crystalline silicon cells, the current generated by
radiation of a particular spectral composition is directly proportional to the
irradiance [Book 2]. Some types of solar cell, however, do not exhibit this
linear relationship. The silicon solar cell has many advantages such as high
reliability, photovoltaic power plants can be put up easily and quickly,
photovoltaic power plants are quite modular and can respond to sudden changes in
solar input which occur when clouds pass by. However there are still some major
problems with them. They still cost too much for mass use and are relatively
inefficient with conversion efficiencies of 20% to 30%. With time, both of these
problems will be solved through mass production and new technological advances
in semiconductors.
Bibliography
Tran 6
Bibliography
1) Green, Martin Solar Cells, Operating Principles, Technology and System
Applications. New Jersey, Prentice-Hall, 1989. pg 104-106 2) Hovel, Howard Solar
Cells, Semiconductors and Semimetals. New York, Academic Press, 1990. pg 334-339
3) Newham, Michael ,Photovoltaics, The Sunrise Industry, Solar Energy, October
1, 1989, pp 253-256 4) Pulfrey, Donald Photovoltaic Power Generation. Oxford,
Van Norstrand Co., 1988. pg 56-61 5) Treble, Fredrick Generating Electricity
from the Sun. New York, Pergamon Press, 1991. pg 192-195
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